Cell phone conversations are prone to abrupt disconnections. When the call is disconnected, both parties are left in a state of not knowing if the other party was done with the conversation.
One party can call the other party back, but there are multiple problems with that. Both parties could try to call each other at the same time, resulting in having to call yet again. Both parties may wait for the other party to call them back. Alternatively, the user could type a text to tell the other party they were done with the conversation.